In recent years, with the improvement of speed and the increase of capacity in optical transmission systems, more and more attention has been paid to an optical phase modulation method represented by Differential Quadrature Phase Shift Keying (DQPSK) modulation. In the DQPSK modulation, four different phases of an optical wave are used for representing different data signals, so a code element speed thereof is only half of that of traditional optical amplitude modulation, therefore requirements on optical devices are reduced a lot. Besides, with more excellent dispersion and polarization mode dispersion performance with respect to the optical amplitude modulation, the DQPSK modulation is better suited to large-capacity and long-distance optical transmission systems.
In the optical fibre communication, an optical carrier can be generally expressed as: Ei=E exp j[ω0t+φ(t)], wherein E refers to field strength, ωo refers to an angular frequency of the optical carrier, and φ(t) refers to a modulation phase. The basic working principle of the DQPSK modulation is: information to be transmitted is encoded in a differential phase, which is expressed by Δφ, of continuous optical bits, wherein Δφ can be any value in [0, π/2, π, 3π/2]. Assumed that the pahse of the k−1th optical bit pulse is θ(k−1), if the adjacent bit is {0, 0}, θ(k)=θ(k−1)+π; if the adjacent bit is {0, 1}, θ(k)=θ(k−1)+π/2; of the adjacent bit is {1, 1}, θ(k)=θ(k−1); if the adjacent bit is {1, 0}, θ(k)=θ(k−1)+3π/2.
In the DQPSK modulation system, a common modulator is a lithium niobate modulator, however, during the practical application, due to the property of the material of the lithium niobate modulator, i.e., relatively high sensitivity to both temperature and stress, in order to realize accurate phase control, certain peripheral control circuits are required. At present, there are two common control methods. The first method is: pilot signals in different frequencies are set on arms I and Q of a lithium niobate modulator, then backlight detection signals are acquired and difference-frequency signals contained in the backlight detection signals are filtered, when the difference-frequency signals disappear, the lithium niobate modulator is locked to a common offset point. The defect of this method is that, as the output amplitude of the difference-frequency signals is very small, it is very difficult to accurately detect whether the difference-frequency signals disappear really, thus the accuracy of controlling the position of the offset point is influenced. The second method is: the backlight detection signals are directly sampled, whether there are Radio Frequency (RF) harmonic signals, which is in the same rate as a data bit stream, contained in the backlight detection signals is judged, when the setup of the offset point makes the RF harmonic signal has the minimum power, it is determined that the current offset point is in a normal locked state. The defect of this method is that the circuit design is relatively complex, furthermore, the implementation cost is relatively high, and the feasibility is relatively low.
No matter which control method is employed to control the offset point of the lithium niobate modulator, the implementation of peripheral control circuits directly influences the performance of the whole DQPSK modulation system, the above two methods have low accuracy of controlling the phase delay offset point of the lithium niobate modulator, as a result, the performance of the whole DQPSK modulation system is degraded.